Update framework testing infrastructure and standards

- Add phpt test runner and suite for app functions
- Introduce testing workflow to AGENT.md
- Add tests for cache, content, context, helpers, hooks, plugins,
  rendering
- Mount tests directory in dev container

devel/tests/content/find_all_content_files.phpt

@@ -0,0 +1,47 @@
+--TEST--
+findAllContentFiles: returns sorted content file paths, ignores non-content files and index.php
+--FILE--
+<?php
+require '/var/www/app/constants.php';
+require '/var/www/app/hooks.php';
+require '/var/www/app/content.php';
+
+$dir = sys_get_temp_dir() . '/phpt_content_' . getmypid();
+mkdir($dir);
+
+// Non-existent directory
+echo count(findAllContentFiles($dir . '/nope')) . "\n";
+
+// Empty directory
+echo count(findAllContentFiles($dir)) . "\n";
+
+// Content files
+file_put_contents("$dir/page.md", '# Hello');
+file_put_contents("$dir/extra.html", '<p>Hi</p>');
+file_put_contents("$dir/script.php", '<?php echo "hi";');
+
+// Non-content files are excluded
+file_put_contents("$dir/styles.css", 'body {}');
+file_put_contents("$dir/image.jpg", '');
+
+// index.php is always excluded
+file_put_contents("$dir/index.php", '<?php');
+
+$files = findAllContentFiles($dir);
+echo count($files) . "\n";
+
+// Results are full paths, sorted by filename (natural order)
+foreach ($files as $f) {
+    echo basename($f) . "\n";
+}
+
+array_map('unlink', glob("$dir/*"));
+rmdir($dir);
+?>
+--EXPECT--
+0
+0
+3
+extra.html
+page.md
+script.php

devel/tests/content/load_metadata.phpt

@@ -0,0 +1,44 @@
+--TEST--
+loadMetadata: parses metadata.ini, returns null when absent
+--FILE--
+<?php
+require '/var/www/app/hooks.php';
+require '/var/www/app/content.php';
+
+$dir = sys_get_temp_dir() . '/phpt_meta_' . getmypid();
+mkdir($dir);
+
+// No metadata file
+echo (loadMetadata($dir) ?? 'null') . "\n";
+
+// Basic key-value metadata
+file_put_contents("$dir/metadata.ini", "title = My Page\nsummary = A summary\nmenu = true\n");
+$meta = loadMetadata($dir);
+echo $meta['title'] . "\n";
+echo $meta['summary'] . "\n";
+echo $meta['menu'] . "\n";
+
+// _raw is always present
+echo isset($meta['_raw']) ? 'raw present' : 'raw missing';
+echo "\n";
+
+// Sections are in _raw but not in base metadata
+file_put_contents("$dir/metadata.ini", "[en]\ntitle = English Title\n\n[fr]\ntitle = French Title\n");
+$meta = loadMetadata($dir);
+echo isset($meta['en']) ? 'section leaked' : 'section contained';
+echo "\n";
+echo $meta['_raw']['en']['title'] . "\n";
+echo $meta['_raw']['fr']['title'] . "\n";
+
+array_map('unlink', glob("$dir/*"));
+rmdir($dir);
+?>
+--EXPECT--
+null
+My Page
+A summary
+1
+raw present
+section contained
+English Title
+French Title

devel/tests/content/resolve_slug_to_folder.phpt

@@ -0,0 +1,48 @@
+--TEST--
+resolveSlugToFolder: resolves URL slug to actual folder name, supports metadata slug override
+--FILE--
+<?php
+require '/var/www/app/hooks.php';
+require '/var/www/app/content.php';
+
+$parent = sys_get_temp_dir() . '/phpt_slug_' . getmypid();
+mkdir($parent);
+
+// Non-existent parent directory
+echo (resolveSlugToFolder($parent . '/nope', 'anything') ?? 'null') . "\n";
+
+// Folder name matches slug directly
+mkdir("$parent/my-post");
+echo resolveSlugToFolder($parent, 'my-post') . "\n";
+
+// Slug not found
+echo (resolveSlugToFolder($parent, 'missing') ?? 'null') . "\n";
+
+// Metadata slug overrides folder name
+mkdir("$parent/2024-01-15-article");
+file_put_contents("$parent/2024-01-15-article/metadata.ini", "slug = article\n");
+echo resolveSlugToFolder($parent, 'article') . "\n";
+
+// Original folder name still works when no slug conflicts
+echo resolveSlugToFolder($parent, '2024-01-15-article') . "\n";
+
+// Metadata slug takes precedence over folder with that name
+mkdir("$parent/other");
+file_put_contents("$parent/other/metadata.ini", "slug = my-post\n");
+// 'my-post' folder exists AND 'other' has slug=my-post — folder name wins (first match)
+echo resolveSlugToFolder($parent, 'my-post') . "\n";
+
+array_map('unlink', glob("$parent/2024-01-15-article/*"));
+rmdir("$parent/2024-01-15-article");
+array_map('unlink', glob("$parent/other/*"));
+rmdir("$parent/other");
+rmdir("$parent/my-post");
+rmdir($parent);
+?>
+--EXPECT--
+null
+my-post
+null
+2024-01-15-article
+2024-01-15-article
+my-post